In this lesson, students learn how a computer represents an image using binary.

Suggested time: 100 mins

Learning Objectives:

Explain the representation of an image as a series of pixels represented in binary

Explain the need for metadata to be included in the file such as height, width and colour depth

Discuss the effect of colour depth and resolution on the size of an image file

Curriculum Mapping:

KS3:​​

Understand how data of various types (including text, sounds and pictures) can be represented and manipulated digitally, in the form of binary digits

Introduction:

Data in computers is stored and transmitted as a series of ones and zeros (also known as Binary).

To store an image on a computer, the image is broken down into tiny elements called pixels. A pixel (short for picture element) represents one colour. An image with a resolution of 1024 by 798 pixels has 1024 x 798 pixels (817,152 pixels).

In order for the computer to store the image, each pixel is represented by a binary value. We call this representation of colours a “bit-plane”. Each bit doubles the number of available colours i.e. 1-bit would give us 2 colours, 2-bits would give us 4 colours and 3-bits would give us 8 colours etc. In a monochrome (two colour) image, like the example below, just 1 bit is needed to represent each pixel e.g. 0 for white and 1 for black.

Images are stored in scan lines. Each line is encoded from left to right, top to bottom. The image here would receive the following binary values:

000000010010010010000000011110000000

In an image that uses 4 colours, 2 bits are needed for each pixel. The following example uses two bits to store the following colours: 00 – White; 01 – Black; 10 – Yellow; 11 – Blue

In this lesson, students learn how a computer represents an image using binary.

Starter

Explain to students that data in computers is stored and transmitted as a series of ones and zeros, also referred to as “Binary”.

Display the following simple 1-bit image on the board and ask students to suggest how it could be converted to binary. Try to draw out answers such as "Use 0 to represent white and 1 to represent black" or "Use 0 to represent black and 1 to represent white".

Explain to students that, in a computer, an image is broken down into tiny elements called pixels and that each pixel (short for picture element) represents one colour. Explain that an image with a resolution of 1024 by 798 pixels has 1024 x 798 pixels (817,152 pixels).

Tell students that, in order for the computer to store the image, each pixel is represented by a binary value (One or Zero). Explain that we call this representation of colours a “bit-plane” and that each bit doubles the number of available colours i.e. 1-bit would give us 2 colours, 2-bits would give us 4 colours and 3-bits would give us 8 colours etc.

Demonstrate this using the 2 colour and 4 colour images in the introduction (above).After showing students how a computer would represent a 2 colour and 4 colour image, ask the students to suggest how we could store more colours such as 16, 256 etc.

Explain to students that an image represented by 24-bits would have 16 million colours - 16,777,216 to be precise.

Finally, hand out the following worksheets and ask the students to copy the image they created in touchdevelop onto the grids provided. Once the students have transferred their images tn their worksheets, instruct the students to convert their image into binary (as if they were a computer).

Students can share the binary code of their favourite bitmap image with a classmate to see if they could convert it back to the original image.

Some students can convert their image to hexadecimal (using the help sheets provided)

Main

Write the following binary example on the board and ask the students how the computer would interpret the numbers.

101001001001101

Note: You can write any binary number on the board - the idea being for the students to ask questions such as how many colours etc.)

Ask students what does a computer need to know to be able to convert the image. Explain that, in order for the computer to interpret the file, the computer needs to know:

• Colour depth – how many bits represent each pixel

• Resolution - Width & Height (in pixels)

Finally, explain that we call this “metadata”

Plenary:

Finish by recapping the learning objectives using the Pose, Pause, Pounce and Bounce method.

Example:

Pose a question to the whole class e.g. Explain how a 16 colour image can be represented in binary

Pause to give students time to digest the question and think of their answer. If the students are engaged, try holding the pause for a little while longer to build up the tension.

Pounce: Quickly, select a student to answer the question. i.e. Insist the answer to the question comes from student A and possibly student B, directly and fast! Obviously, plan in your mind who you are going to direct the questions to before hand.

Bounce the question or student's response on to another student (immediately after the pounce). e.g. Ask them if they agree with the students previous answer and to explain why.